This invention relates to apparatus for burning cement raw meal to produce cement clinker and in particular to a combined suspension preheater, flash calciner, rotary kiln, and clinker cooler for producing cement clinker. The apparatus is particularly useful where one desires to initially construct a clinker producing plant of a given capacity and then at some later date expand the capacity of that clinker producing plant without the necessity of adding a complete separate system.
Prior to the present invention it was known to produce cement clinker in a rotary kiln using a grate type cooler wherein cooling air is passed upwardly through a bed of hot clinker to cool the hot clinker and supplying to the kiln as preheated combustion air some of the cooling air which is heated by the hot clinker. It was also known prior to the present invention to use in the dry process for producing cement clinker a gas-solids heat exchanger which uses the exhaust gases from the rotary kiln for preheating the cement raw meal before it is fed to the kiln. One type of gas-solids heat exchanger is a suspension preheater which includes a plurality of serially flow connected cyclone separators which receive cement raw meal at the top thereof and exhaust gas from the kiln at the bottom thereof with generally counter-current flow of the hot gas and raw material through the preheater to thereby preheat the raw feed for the kiln.
The preheater operates by supplying raw meal to a stream of hot gas, entraining the meal in that hot gas, transferring heat from the hot gas to the raw meal, separating the hot gas and raw meal in a cyclone separator, supplying the raw meal to another stream of hot gas for entrainment and subsequent separation. A typical system may include four cyclone separators to achieve four stages of heat exchange.
A more recent development in the art is the employment of a separate calcining furnace or flash furnace for receiving preheated raw material from the preheater, subjecting that material to a temperature on the order of 900.degree. C for a brief period of time, perhaps 1 or 2 seconds, to accomplish about 95% calcination of the raw meal and then supplying this calcined raw meal to the rotary kiln for clinkering. The addition of the flash furnace results in increasing the capacity of a given size kiln by about two times. The reason for this is that the rotary kiln now only performs the clinkering phase of producing cement clinker rather than both the calcining and clinkering phase. A system such as the above is shown, for example, by U.S. Pat. No. 3,869,248.
In many instances, a cement producer will desire to construct a new plant of a given capacity which may suit his current needs as well as his current budget. For example, a cement producer may desire to install a clinker producing plant having a production capacity of 1,000 tons per day. The most modern method of producing cement at 1,000 tons per day may be a dry process cement plant with a suspension preheater. Accordingly the cement producer may install such apparatus. At some future date, the cement producer may require say 2,000 tons per day production. The producer has a choice of installing a completely separate rotary kiln and suspension preheater clinker producing system, or by the present invention, adding a flash furnace and additional suspension preheater system to his existing clinker producing system.
With the use of pre-calcining, an up to two fold increase in capacity can be achieved while using the same size rotary kiln. An additional suspension preheater is required because the existing suspension preheater will not be able to reasonably handle the full capacity of the new system. The material cooler will also have to be expanded in size and the kiln speed increased due to the additional clinker produced.
Various dual preheater systems employing a flash calciner and rotary kiln clinkerer are known prior to the present invention. Such systems are shown in U.S. Pat. Nos. 3,864,075 and 3,881,861 and Japanese Utility Model Application No. S 47-19826, filed Feb. 17, 1972. The use of a two parallel suspension preheaters with two parallel flash furnaces each being supplied raw meal from its associated preheater and both feeding calcined raw meal to the single clinkering kiln is, of course, a possible solution, but such a system would be more costly as two flash furnaces would be required meaning two vessels and associated auxiliary equipment and would not be as thermally efficient as a single flash furnace. The use of a single flash furnace and a dual final cyclone which supplies spent combustion gas to both suspension preheaters heretofor presents the problem of insuring equal gas and material distribution to the two suspension preheaters. A balanced gas distribution can be achieved through the control of the separate preheater fans, but the division of material exiting the flash furnace to dual cyclones is difficult to control. Therefore, a single calcining furnace and single final cyclone is herein proposed.